1. Technical Field
The present invention relates generally to retroreflectors, and more particularly, to modulating retroreflectors useful for returning a encoded signal to the source of an optical interrogation beam.
2. Description of the Background Art
Modulating retroreflectors were demonstrated before the invention of the laser, but were restricted to short distances and low data rates. The first disclosure of a modulating retroreflector appears in a 1903 British patent, No. 21,856 titled “Improvements in and Means for Signaling and Indicating Position of Objects” to Sir Howard Grubb. Harry Stockman, “Communications by Means of Reflected Power,” Proceedings of the IRE, pp. 1196-1204, October 1948, provides another early description of a modulating retroreflector for free-space optical communications.
U.S. Pat. No. 4,361,911 to R. G. Buser et al. describes a laser retroreflector system suitable for identifying whether a target is a friend or foe in a battlefield situation.
In the 1990's modulating retroreflectors were developed that allow free-space optical communication between a node with minimal power, weight, and pointing ability and a node with higher power, weight, and pointing ability. For example, U.S. Pat. No. 6,154,299 to G. Charmaine Gilbreath, Steven R. Bowman, William S. Rabinovich, Charles H. Merk, and H. E. Senasack, “Modulating retroreflector using multiple quantum well technology”, the entire disclosure of which is incorporated by reference herein, describes a modulating retroreflector developed at the Naval Research Laboratory. These modulating retroreflector systems are particularly suitable for communications links between aircraft and ground stations, because most of the weight and power requirements are relegated to the ground-based interrogator station, allowing the aircraft's modulator to be small, light, and low-power.
There are two basic classes of retro-reflectors, “cat's eye” and corner cube retro-reflectors. “Cat's eye” retro-reflectors combine lenses and/or mirrors and incorporate an optical focus. Several variations of cat's eye retro-reflectors are described in Mark L. Biermann et al., “Design and analysis of a diffraction-limited cat's-eye retroreflector,” Opt. Eng., Vol. 41, pp. 1655-1660, (2002). In contrast, corner cube retro-reflectors (CCRs) are nonfocusing. Some tradeoffs between modulating retro-reflectors of the corner cube type and the cat's eye type are discussed in P. G. Goetz, W. S. Rabinovich, R. Mahon, L. Swingen, G. C. Gilbreath, and J. Murphy, “Practical Considerations of Retroreflector Choice in Modulating Retroreflector Systems,” IEEE LEOS 2005 Summer Topicals, San Diego, Calif., 25-27 Jul. 2005.
Recent advances in optoelectronic devices and free-space optics have greatly increased the capabilities of modulating retroreflector systems. Examples are discussed in “Peter G. Goetz, William S. Rabinovich, Rita Mahon, Mike S. Ferraro, James L. Murphy, H. Ray Burris, Mena F. Stell, Chris I. Moore, Michelle R. Suite, Wade Freeman, G. C. Gilbreath, and Steven C. Binari, “Modulating Retro-Reflector Devices and Current Link Performance at the Naval Research Laboratory,” MILCOM 2007, Orlando, Fla., October 2007” and Peter G. Goetz, William S. Rabinovich, Timothy J. Meehan, D. S. Katzer, Steven C. Binari, Eric E. Funk, G. Charmaine Gilbreath, Rita Mahon, Lee Swingen, John Rende, Eugene Waluschka, Gary Lepore, and Anthony Phan, “Modulating Retroreflector Implementation of MIL-STD-1553 Protocol with Free-Space Optics”, Proceedings of the 2003 IEEE Aerospace Conference, Paper No. 1559, 2003.
U.S. Pat. No. 7,719,746 to Goetz et al., and P. G. Goetz, W. S. Rabinovich, S. C. Binari, and Mittereder, “High-Performance Chirped Electrode Design for Cat's Eye Retro-Reflector Modulators”, IEEE Photonic Technology Letters, vol. 18, No. 21, Nov. 1, 2006, pp. 2278-2280, describe a chirped electrode for use in a multiple quantum well modulating retroreflector. Gridded electrodes for use in solar photovoltaic cells are discussed in H. B. Serreze, “Optimizing Solar Cell Performance by Simultaneous Consideration of Grid Pattern Design and Interconnect Configuration,” in the Conference Record of the IEEE Photovoltaic Specialists Conference pp. 609-614, 13th IEEE PVSC, pp. 609-614, 1978, and in A. R. Burgers, “How to design optimal metallization patterns for solar cells”, Progress in Photovoltaics: Research and Applications; Vol. 7, No. 6, pp. 457-461, 1999.
U.S. Pat. No. 7,715,727 to Murphy et al. describes a system and method for transmitting analog signals with a modulating retroreflector using hybrid amplitude and frequency modulation. In W. S. Rabinovich et al., “45 Mbit/s cat's-eye modulating retroreflectors”, Optical Engineering, Vol. 46, No. 10, pp 104001-1-104001-8, October 2007, describes various MQW-MRR optical communications systems configured as cat's-eye retroreflector systems with the multiple quantum well located in the focal plane of the cat's eye optic.
U.S. Patent Publication Number 20070297801A1, to Rabinovich et al., describes an optical communication system with a cat's eye modulating retro-reflector (MRR) assembly. The system includes a beam deflector for decreasing the field of view of the retroreflector, and can include a separate angle of arrival sensor for sensing the arrival angle of the interrogating beam in order to select which modulator pixels to activate.
L. D. Westbrook and D. G. Moodie, in “Simultaneous bi-directional analogue fibre-optic transmission using an electroabsorption modulator”, Electronics Letters, Vol. 32, No. 19, pp. 1806-07, September 1996, discuss using a multiple quantum well electroabsorption modulator as both a photodetector and a modulator in a frequency-division-multiplexed analog fiber optic system.
In some previous systems having a number of multiple quantum well pixels, all of the pixels would have been driven whether or not they were illuminated. This configuration generated a lot of heat and required a high power level. In other designs, a separate pixel array has been used to identify which modulator pixels are illuminated, in order to select the corresponding drivers so that only the illuminated pixels are driven. In this case, the angle of arrival sensor includes an array of reverse-biased photodiodes and a second set of optics, with the second optics being precisely aligned to the primary cat's eye optic, and a carefully calibrated correspondence between the photodiode array and the modulator array over a wide range of input angles. See, for example, the system described in W. S. Rabinovich et al., “45 Mbit/s cat's-eye modulating retroreflectors”, Optical Engineering, Vol. 46, No. 10, pp 104001-1-104001-8, 2007.